Lately, I've come to realize that like standard mainstream anime OVA actually, in fact, have their own genres.

1. The alternate ending(s) - sometimes this is a writers decision to give his true ending he wanted because the studio wanted something against his vision. Sometimes it's just what it sounds like, an alternate ending. Yet, sometimes an author just realizes he ticked off his viewer-base enough to warrant a new ending.(Future Diary was the best example of this).

2. The unaired episode(s) (in show timeline) - an extra story just to tell another chapter of the story to the fans who follow it. Angel Beats had 2).

3. The unaired episode(s) (outside show timeline - a story which takes place either as a prequel or sequel to the series, a solo story involving a character of the story or a bridging OVA to introduce characters for a new upcoming season. (The Seven Deadly Sins Ban OVA and upcoming Tokyo Ghoul OVAs are the best examples of this).

4. The lets give our fans a special ecchi/hentai episode with their fave characters episode(s) - self-explanatory. (The R-15 and Yamada's First Time OVAs are good examples of this).

5. The there isn't enough demand for a full series "exclusive" OVA(s) - It could be one episode, could be a dozen but they come out on a very irregular schedule (maybe ep 2 6 months after ep 1 or ep 3 2 years after ep 2) or be a pre-arranged short run of only 1-4 episodes, (Yamada-Kun and the 7 Witches had a one-shot years prior to their full anime series, Denpa Teki na Kanojo was a pre-arranged 2 episode run).

What is your favorite genre? Which do you dislike most? Did I miss any style/genre of OVA you can think of?

For me, my favorite would have to be alternate ending OVA and my least favorite in show timeline unaired episodes.